There are several reasons for keeping an animal leash under a slight tension at all times. First, an animal tied to a leash in a backyard for example, moves around and back and forth within the leash's range. When the leash has slack segments in it, it tends to become entangled around its support structure and around nearby objects. More important, the leash can become entangled around one of the animal's legs or other body part, and can entrap the animal in a precarious position for long periods of time. A slight tension in the leash keeps it free of knots and snares. A second reason for keeping an animal leash under a slight tension is to lessen the impact of a tight-leash stop on the animal's collar.
In the past, several devices were developed to apply a slight tension on a leash or on other tethering cords used for retaining horses, boats and even for preventing a child from wandering away from a supervised playground.
Examples of these prior art devices are described in the following U.S. patents. These documents are divided in three groups. The documents in the first group describe tethering cords passing over a sheave or a similar rope guide mounted in a vertical support structure. In each case, a single weight is suspended to the cord inside the vertical structure. As the cord is pulled away from the vertical structure, the weight is lifted, thereby applying a tension on the cord. This first arrangement is described in the following patents.    U.S. Pat. No. 124,532 issued to C. F. Barnard on Mar. 12, 1872;    U.S. Pat. No. 167,263 issued to T. Maguire on Aug. 31, 1875;    U.S. Pat. No. 183,677 issued to J. L. Jackson on Oct. 24, 1876;    U.S. Pat. No. 750,141 issued to W. J. Willey on Jan. 19, 1904.
One improvement to the above-described arrangement has been to incorporate a compression spring around the cord, between the rope guide and the weight, to absorb the impact of the weight reaching the end of its travel. This type of tethering device is illustrated in the following documents:    U.S. Pat. No. 757,882 issued to T. C. Butterworth on Apr. 19, 1904;    U.S. Pat. No. 3,189,004 issued to F. Sinclair on Jun. 15, 1965.
In yet another improvement to tethering devices, a mechanical advantage has been introduced in the restrainer portion of each device. A pulley block is attached to the weight and the cord forms a loop that is threaded into the pulley block with its fixed end tied to the upper end of the vertical structure. The weight is thereby suspended to two segments of the cord whereby the total cord extension is twice as long as the weight's travel distance along the vertical structure. This type of tethering device is illustrated in the following patents;    U.S. Pat. No. 215,730 issued to J. A. Fox et al. on May 27, 1879.    U.S. Pat. No. 550,620 issued to J. C. Jones on Dec. 3, 1895;    U.S. Pat. No. 2,725,853 issued to J. Nordheim on Dec. 6, 1955;    U.S. Pat. No. 4,747,372 issued to M. Terry et al. on May 31, 1988.
The system described in U.S. Pat. No. 550,620 in particular, differs slightly from the other devices in the latter group because it has two loops in series along the cord and two weight-and-block arrangements respectively mounted in these loops. One of the weights is heavier than the other, and is lifted last for increasing the pulling resistance on the cord incrementally.
It is believed that when one discrete change in tension is applied to a tether rope at a specific location, or when two changes in tension are applied to a tether rope at a fixed distance interval between each other, an animal can learn the locations where the tension increases in the rope along the rope's range. An animal tied to the rope can thereby anticipate and get accustomed to these changes in tension, and can still run to the end of the rope with considerable momentum.
Although the tethering devices of the prior art deserve undeniable merits, it is believed that a need still exists for a tethering device which is more efficient than the prior art installations for dissuading an animal from attempting to reach the end of its leash.